Thermocolor ribbons have long been known. Generally, they comprise a preformed foil-like carrier, for example, of paper or plastic onto which the color transfer layer with the melt color is applied. The latter can comprise a meltable wax-bonded or plastic-bonded coloring agent or carbon black layer.
The melt color, upon the use of such thermocolor ribbons, can be melted by the action of a thermal printing head to transfer the symbols, which can be alphanumeric characters, to the substrate which may be a receiving paper or foil. In general, such ribbons are referred to as thermal transfer ribbons or "TCR" ribbons (thermal carbon ribbons). Thermal printers which utilize heat to transfer a symbol to a substrate with such ribbons are known, for example, from German Patent documents DE-AS 2,062,494 and DE-AS 2,406,613 as well as DE-OS 3,224,445.
During the printing process with a thermocolor ribbon, the following operations generally take place: the printing head of a thermal printer presses the thermocolor ribbon against the receiving substrate. The printing head develops a temperature which is sufficient to melt the melt color, but generally is a maximum of about 400.degree. C.
The uncoated backside of the thermocolor ribbon, namely, the foil-like carrier remains during the printing process in direct contact with the print head or the hot print symbol, e.g. a typeface. In the relatively short time required for the printing process, the relative speed between the thermocolor ribbon and receiving paper or foil is zero. The coating on the carrier is melted in the pattern of the symbol and is transferred to the receiving substrate when the pressure from the print head is withdrawn, the transferred melt color remain adherent to the paper as the ribbon is pulled back therefrom.
Mention may also be made of another process which also uses thermocolor ribbons, but wherein the symbol transfer is not effected by the use of a heated symbol of the printing head, but rather is a consequence of resistance heating generated in a special foil-like carrier. The melt color used forms a functional layer which is locally brought to a temperature sufficient to melt the layer by resistance heating for transfer of the symbol. Since the ribbon is electrically conductive, in the field the process is referred to as an electrothermal process and the ribbon as an electrothermal ribbon. Such a thermal transfer printing system is described, for example, in U.S. Pat. No. 4,309,117.
Commercially available thermocolor ribbons at the present time primarily make use of a foil-like carrier which is constituted of a polyester or polycarbonate as a rule. The provision of such carriers involves high material costs. Recycling of the foil after use of the ribbon is impractical.
As a consequence, it has been proposed in European Patent Publication EP-A-0 120 230 to provide a thermal color ribbon of a special structure such that no carrier foil is required and thereby the ribbon can be manufactured at low cost.
In this system, the melt color is composed of a first hard polymer, a film-forming material, a second polymer which is an adhesive or an agent of low melting point, and a color releasing material.
This thermocolor ribbon must have sufficient tensile strength to allow it to be self-supporting and pulled along the ribbon path. It has been found in practice, however, that it is necessary to support this ribbon along its path by an endless belt. The endless belt, of course, is an additional mechanical element so that apparatus is made more costly and, naturally a special printer must be used.